We have set up a 1 photon FPR rig that can measure the mobility of membrane probes on cell membranes. This data will be compared to single particle tracking data of the same probes. We are interested in determining the identity and function of surface proteins and other molecules that are involved in the aggregation of antibody receptors on the surface of mast cells. Recent data suggest the existence of membrane domains where co-aggregation of several membrane components, such as cholesterol, certain gangliosides, and the receptor itself, can occur during the triggering mechanism. By measuring diffusion coefficients of these components under different conditions and at different times in the activation process on RBLs, we hope to lend further credence to such domains' suggested existence as well as investigate their properties, whether they pre-exist on the cell's surface, their size and coverage of the cell, and their role in the signaling process.